Li metal batteries promise the next-generation rechargeable batteries for electric vehicles applications due to the highest specific capacity (3840 mA h g^-1) and lowest reduction potential (−3.04 V vs. S.H.E.) of Li among all alkali and alkaline earth metals. However, a non-uniform solid electrolyte interphase (SEI) layer formed via parasite reactions of Li with the electrolytes leads to locally site-specific Li deposition, i.e., dendritic plating.¹ Dynamic breaking and forming of this layer, in turn, leads to the accumulative loss of Li, build-up of the SEI and consumption of the electrolyte causing cell failure. Upon prolonged cycling, penetration of dendrites through the separator causes hazardous cell short-circuits. Modified electrolytes including concentrated electrolytes,² LiF additive³ and artificial SEI layer⁴ have shown promise in suppressing dendrites.

In this presentation, we will demonstrate an in-vivo formed sulfide-based ionic conductive layer — realized by using a low concentration complexed electrolyte additive — that not only generates a kinetically favorable interface, but also reduces the corrosive reactions of the electrolyte with Li. Upon Li plating, the high ionic conductivity of the solid electrolyte layer allows the Li⁺flux to be evenly distributed over the conductive surface. Furthermore, even at extremely high current densities when incipient dendritic Li start to form, the reservoir additives in the electrolyte facilitate local formation of the conductive layer.

Upon resting at OCV, the Li|Li symmetric cell in the presence of the electrolyte additive shows much lower interfacial charge transfer resistance than a cell with a “blank” electrolyte (20 vs. 320 Ω cm^-2). Furthermore, the impedance is stable over 2 days of rest, in contrast with significantly increasing impedance exhibited by the blank cell. Greatly improved long-term cycling of the symmetric cells is observed. The cell with additive exhibits extremely stable voltage evolution over 400 hours, whereas the blank electrolyte cell experiences voltage fluctuation followed by short-circuit after 270 hours (Figure 1a). Notably, stable cycling of Li|Li symmetric cells over 2500 hours at 1 mA cm^-2 with 1 mA h cm^-2 capacity was achieved using 100 mM additive (Figure 1b), and a significant decrease in polymerized solvent is observed compared to a cell with no additive. In a full Li metal cell using Li₄Ti₅O₁₂ as the positive electrode, the cell with the electrolyte additive exhibits very stable capacity retention (88%) over 400 cycles.